When plaque deposits in blood vessels obstruct flow, the plaque blockage is commonly surgically bypassed by extracting a little used blood vessel from the patient and connecting it to a source of blood above the blockage and to the vessel below the blockage, thus creating a bypass for blood to flow around the obstruction. However, bypass surgery entails significant morbidity and mortality, as well as substantial cost. Alternatively, in a procedure called balloon angioplasty, a catheter with a balloon mounted on the distal end thereof can be guided into and expanded within the blockage, to compress and, perhaps, fracture the plaque, thus permitting blood flow through the vessel to resume at least partially. However, the blockage typically recurs after balloon angioplasty procedures in up to 40% of the patients within six months.
Alternatively, excimer laser energy, which is capable of vaporizing tissue with only minimal thermal effect, can be transmitted through fiber optic catheters, such as the catheter described in U.S. Pat. No. 4,732,448 to Goldenberg, and can be used to vaporize plaque deposits in blood vessels. However, many vessels are not treatable with excimer laser catheters, either being too large, too tortuous to traverse or too completely blocked to admit a guidewire over which the catheter may be advanced. In vessels which can be treated with excimer laser catheters, the blockage recurs in up to 40% of the patients within six months. Likewise, mechanical atherectomy devices, such as rotating burrs and cutting devices, have been employed in vessels blocked by plaque, but the clinical results of mechanical atherectomy devices have been no better than balloon or excimer laser angioplasty.
Plaque deposits in blood vessels can also be treated by using stents, i.e., tiny coils of stainless steel or other materials, which are deposited within a ballooned or layered plaque deposit to help keep the vessel open to blood flow. However, blockages in stents recur in up to about 15% to 25% of the cases within six months, due to the in-growth of tissue in the vessel opening. Effectively reopening a blocked stent is often difficult or impossible.
In the late 1980s, Argon and Nd:YAG laser energy was used to melt and vaporize plaque in blood vessels. The laser energy was transmitted through an optical fiber whose distal end was encased in a bulbous metal cap, usually made of stainless steel, and which was rapidly heated to 400° C. or more in a few seconds by the laser energy, as described in co-owned U.S. Pat. Nos. 4,646,737, 4,662,368 and 4,773,413, which are fully incorporated herein by reference.
These laser devices, commonly referred to as “Hotip”™ devices, were able to make relatively smooth channels through vessel plaque deposits. However, in substantially occluded blood vessels with little blood flow to cool the Hotip™ device, temperatures often reached 800° C. or higher. Even at a temperature of 400° C., thermal energy radiating sideward from the metal tip could damage the wall of the blood vessel underlying the plaque if the device was not kept rapidly moving back and forth in the vessel during the lasing procedure and, after switching off the laser, for 5 to 10 seconds, while the device cooled. If the Hotip™ device remained static in a blood vessel, severe damage would occur.
When properly used, the initial success rate of the Hotip™ device in peripheral (leg) vessels was 89% and, over a period of 30 months, was comparable to the results of peripheral artery bypass surgery. However, in the early 1990s, sales of these devices declined sharply, after the publication of several reports of injuries to blood vessels in humans, mainly due to lateral radiation of heat and damage to the arterial wall as the result of the operator's failure to keep the tip moving back and forth during and after the lasing procedure. Hotip™ devices and their associated lasers, which enjoyed sales of more than $30 million in each of 1989 and 1990, are no longer sold in any substantial amount for this purpose.
Another laser device which has been used to vaporize plaque deposits includes an optical fiber with a metal fitting attached to its distal end which in turn is connected to a second metal fitting holding a lens of sapphire or synthetic sapphire, such as described in U.S. Pat. Nos. 4,592,353 and 4,693,244 to Daikuzono. With these devices, however, heat has a tendency to develop at the junction of the optical fiber and the sapphire lens due to scattering of light energy at the junction. This, in turn, heats the metal fittings and causes lateral radiation of thermal energy. In addition, such devices are expensive to manufacture and costly to the hospital.
Because of the various disadvantages associated with the use of the above laser devices used to treat occluded blood vessels, it would be desirable to be able to melt and vaporize plaque deposits safely in blood vessels and in a minimally invasive manner with an alternative simple, reliable, inexpensive, non-metallic device, which would not thermally damage the vessel wall underlying the plaque deposit and avoid the risks and cost of bypass surgery and the high occlusion recurrence rates currently associated with the use of balloon and excimer laser angioplasty and mechanical atherectomy devices.